Giorgio Querzoli

The investigation of an unstable convective flow using optical methods

The convective flow field in a vessel is investigated by laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV, namely Particle Tracking Velocimetry—PTV). The vessel is heated from below along a linear element at a temperature higher than that of the fluid. Hot fluid raises up and generates two counterrotating vortices. For a given aspect ratio, the two vortices become unstable and start to oscillate on a vertical plane (orthogonal to the heating element). This regime is investigated for increasing Rayleigh numbers to analyze the transition from regular to irregular conditions. The main transition mechanism is observed to be mostly connected to type II intermittency, a mechanism not frequently observed in experiments. However, at some Rayleigh numbers the present data does not definitely rule out type III intermittency. The phenomenon is analyzed by looking at the main frequencies in the spectrum of the horizontal velocity component and their changes with the Rayleigh number at a point above ...

PTV and POD analysis of the instabilities in a quasi two-dimensional convective flow

An investigation is made of the motion of fluid in a transparent rectangular vessel, heated beneath by a line source, using Particle-Tracking Velocimetry. The control parameters are the Rayleigh number (computed by the power supplied to the system), the Prandtl number and the aspect ratio (height to width ratio) of the vessel. When convection starts, two counter rotating rolls occur. For certain values of the control parameters, the flow becomes unstable and the rolls start to oscillate on a plane orthogonal to the line source (natural swaying motion). The Lagrangian description of the velocity fields (i.e. particle trajectories), is extracted. Data are converted from Lagrangian to Eulerian framework and the time evolution is analysed. Perturbations and instabilities of the velocity field (due to thermal anomalies), are observed. They are related to the natural swaying motion of the rolls. The Fourier decomposition is applied in several points of the grid to obtain a spatial portrait of the energy distribution in frequency domain. By means of Proper Orthogonal Decomposition a complete set of orthonormal functions (modes) is evaluated and the flow field is reconstructed using a small number of modes: POD theory states that these modes retain a greater amount of energy than that of any other possible decomposition. Reconstructed velocity fields and the original ones are compared. In this experiment the first eight modes are found to retain more than 80% of the flow kinetic energy.

Application of High Performance Computing to PIV Technique for Velocity Measurements in Flows.

ABSTRACT Within the ESPRIT Programme the PATRANS (“PArticle TRAcking Neural System”) project (E.P. 7504) has been funded by CEC in order to develop a new system based on a parallel computer (QUADRICS) for the real time measurement of velocity inside flows. The velocity measurements are obtained by applying image processing algorithms to the images of the framed flow field. The system also uses a neural network for the classification of image characteristics useful for velocity measurement.